text:
Why does the military not use Engine Condition Trend Monitoring?
What are the advantages of ECTM and why is it not used in military aviation even though it will help in many ways?

Who says it isn't @Thomas1234? What are you basing this on?

https://apps.dtic.mil/dtic/tr/fulltext/u2/p014128.pdf describes engine monitoring, including engine health monitoring, for the Eurofighter.

Which country's military are you referring to?

ECTM is, I would argue, a necessity for proper maintenance planning/efficiency. I'm not sure why you think it isn't used in military aviation, and I suppose I cannot answer for every military aircraft fleet in the world, but I can assure you that it is used in military aviation.
My answer may not be very complete, but the very short version is "If you don't measure something, you can't plan for it or make it better".

The main advantage is establishing a normal rate of degradation for the engines so that you can pick up on anomalies that suggest latent problems, as well as anticipate major overhaul points with better accuracy which is great for long term planning.
Three of the biggest items to monitor would be ITT margin (the difference between  the peak ITT shown at takeoff thrust and the max ITT - when you are out of margin you are out of engine), oil consumption, and vibe levels.
As Capt Reynolds shows, I would expect that trend monitoring is common with penny pinching NATO airforces, but to the extent that military organizations don't use trend monitoring, I would guess that it's due to a "disposable" culture within the organization.


text:
How can drag induced by the landing gear be reduced?
What are some of the methods used to reduce the drag caused by the landing gear for aircraft with non-retractable landing gear?

A flying boat's landing gear imposes no added drag to the airframe. (Sorry, couldn't resist that one!) :-D

The Pilot's Manual: Ground School has a nice diagram that explains it pretty well. It's all about streamlining. Airplane designers add landing gear fairings (I've heard them call "wheel pants") over top of the wheels and also on the landing gear strut. Beyond this, there isn't much else to do. The fairings could be made out of single pieces of material to limit the parasite drag introduced by rivets, etc.

There can be different variations on the fairing, but it pretty much comes down to fairings to reduce the drag:

NACA (precursor to NASA) details more in REPORT No. 518 THE DRAG OF AIRPLANE WHEELS, WHEEL FAIRINGS AND LANDING GEARS II-NONRETRACTABLE AND PARTLY RETRACTABLE LANDING GEARS
They also looked at wheel shape, but it primarily comes down to creating some sort of streamlined fairing to reduce the amount of eddying.

Paulo Iscold gave a presentation at our EAA meeting last month where he talked about all the ways he investigated to break speed records for piston airplanes. One of the more interesting things that hasn’t yet made into a flying aircraft is to mount electric turbines on the fairings.

Here is one way-- no longer in fashion-- I believe these are called "spats"
http://www.airminded.net/alpha/gamma_skch_flt.jpg
from-- and see here for more-- http://www.airminded.net/alpha/gamma.html

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Can quadrotors be used for commercial passenger flights?
Recently, I have been conducting research on more efficient means for vertical lift transportation. One direction I was thrown in was the use of a quadrotor system  with vector thrust and artificial stability. All of this is done using a system of ducted fans and electric motors. I would love your answer on "Does a system like this sound like a safe and quiet means to transport passengers?"
The name of the program is UV-4, and although it is being tested on flight simulation as an unmanned cargo delivery craft, I wonder if it also makes sense as a passenger aircraft.

Would this type of aircraft produce less noise than a traditional transport category helicopter of the same size? Length = 41ft (Prop blades are ducted and aircraft has two electric motors)
Can a ducted fan rotorcraft create regenerative energy using magnets or other method to create square waves?
This experimental is a UAV; could you picture a piloted passenger aircraft?

Comments are not for extended discussion; this conversation has been moved to chat.

The three most important factors in passenger air travel are safety, safety & safety. So that is where most of the questions will focus on, and where most of the engineering effort will be put into. Couple of questions:

Artificial stability: what happens if the stability circuit breaks down?
What happens if one of the motors fails, for instance one of the big aft ones? Can the craft still stay upright or does it topple over? If it stays upright, can the remaining motors still provide enough lift to keep the craft flying?
What happens if the battery runs down or fails during flight?

These are redundancy problems that need to be solved. Any system that causes an unsafe situation when it fails must have a redundant layout. So at least a penticopter instead of a quadcopter: upon engine failure, 4 props can safely land the craft.
The redeeming feature for quadcopter is ease of manoeuvring with variable rotational speed per propeller, instead of having to make a swash plate arrangement. But light single rotor helicopters have been around for a long time and the technology is mature - I don't see what particular problem the quadcopter can solve that for instance an R44 might have.
On your questions:

Q1 Noise: yes electric motors are nice and quiet, and the shrouds shield noisy tips in most directions.
Q2 Regenerate energy: it can only do that during authoritative descent, which would be frighteningly fast due to the relatively small total rotor disk area.
Q3 Not a UAV: sure, tilt rotors exist already

Edit: the propulsion system you draw is not so revolutionary. If you travel to Niagara Aerospace Museum, you can see the X-22, built in 1967.

From a comment:

•I have had one rotor fail (larger) and be able to land the aircraft safely, even maintain a hover but not climb. Aircraft does not topple in simulations, software used Xplane 10 and 11 ultra releastic. – Robert Gomez

That is a claim that sounds hollow. It depends on the location of the centre of gravity: if it is within the triangle formed by the remaining three rotors, it will stay upright. Picture an X cross connecting fwd left - right aft, fwd right - left aft.

If CoG is behind the middle point of the cross, either one of the front rotors can fail and the craft will fly stable on the remaining rotors, because CoG is within the triangle formed by the operational rotors. But not when one of the aft rotors fail.
If CoG is forward of cross centre, either one of the aft rotors can fail. But not a front rotor.
If CoG is exactly at cross centre, the rotor diagonally opposite the failed rotor cannot exert thrust otherwise the craft will roll. So one rotor fails means only two can be operational.

Conventional rotors have known limitations and are known to be vibration prone. Smaller rotor reduce aircraft vibration and vector thrust gives a vertical lift vehicle the ability to join the super manuerverabily class of aircraft.

•artificial stability is an added feature, the aircraft does fly without it. It is designed to assist the pilot in adverse conditions and gives the pilot a near perfect hover and stable flight condition.

•I have had one rotor fail (larger) and be able to land the aircraft safely, even maintain a hover but not climb. Aircraft does not topple in simulations, software used Xplane 10 and 11 ultra releastic.

•Battery fail means auto rotation and glide ratio.

@RobertGomez current airworthiness standards require multi-engine aircraft to be able to climb with a failed critical engine. Furthermore auto-rotation is not a catch-all, as Koyovis pointed out, it would have to be done at significant speeds for a multi-rotor design like this one. In addition, super-maneuverability, while a fine buzzword, is not something pax tend to enjoy :)

@RobertGomez I'd like to know what glide ratio you envision that body to have, even a rough approximation.

The V22, a work of art that you wouldn`t like to use for your daily commute, related SE: https://aviation.stackexchange.com/questions/22491/can-the-v-22-autorotate

@Caterpillaraoz If the V22 has trouble with autorotation in helicopter mode, I really would like to see the glide ratio of the quad rotor, with its much smaller rotors and inertia.

@RobertGomez Please note that we're not a discussion forum. Comments are for requesting clarifications to answers and pointing out problems, not for carrying on discussions.

@Koyovis quadcopter basically cannot autorotate with such low-inertia props and what we may end up seeing in transportation are 5-6-7 rotors, not 4

@RobertGomez I don't think you understand what autorotation is. Autorotation is converting height (potential energy) into rotor rotation, storing it there in rotor inertia and then spending that energy to cushion landing. Small rotors have very small inertia and quadrocopters by definition require as small inertia as possible (because rotor speed is the only flight control mechanism). Therefore, quadrocopters cannot autorotate by definition.

Moller M400 Skycar is another example of quad tilt duct, although not nearly as successful as X-22. Uses 2 engines per nacelle to handle single motor failure.

What are the advantages that quadrocopters have over conventional helicopters?

Super cheap flight control achieved via electronic control of individual electric motors. (Basically, it means avoiding the mechanically complex main rotor hub.)
Self-cancelling torque thanks to counter-rotating propellers, further simplifying construction.

Now, let's look at the disadvantages:

Quadrupled points of failure - any motor goes down, entire craft goes down.
Abysmal power-to-weight ratio due to electric propulsion and power storage. That's why quadrocopters have flight times of around 5 minutes.
Crappy efficiency due to few small rotors instead of one big.

To sum it up: quadrocopters trade safety and economy of operation for economy of manufacture.
Now, what qualities are important for commercially operated craft, especially ones that carry passengers? First is safety. Then economy of operation. Price of the craft is not very relevant - if it can be operated cheaply enough, the initial investment will pay off sooner or later.
Conclusion: quadrocopters are the worst possible solution for commercial operation, particularly ill-suited to carrying people. The only imaginable commercial scenario is when the operation involves very high risk to the craft. Real life validates this claim: so far the only successful applications of quadrocopters are high-risk drug delivery, drone racing and toys - in all those, cost of acquiring the aircraft dominates.
BTW, your project is not a pure quadrocopter in the common understanding of the term. The thrust vectoring adds complication. You have already traded some cost of manufacture to get better operating efficiency. Follow that trend, and you'll eventually reach the typical helicopter layout with a combustion engine. Or tiltwing/tiltrotor if you need very long range.

FTR, the bad power-to-weight ratio is not inherent to electric propulsion but to batteries. If the main source of electricity was a turbine-driven generator, this wouldn't be so much of an issue anymore. Of course the generator and motors would still be dead-weight compared to a conventional chopper.

@leftaroundabout yeah, I was comparing whole package to whole package, including tanks / batteries.

An important point is that that type of aircraft can't glide in autorotation, as helicopters are able to do in case of engine trouble. It will drop like an stone...

This has been one of the most interesting subjects to investigate and study with this airfoil. Turns out the aircraft can conduct a non power glide for quite a distance with its lifting properties. The idea is to let air rush the ducts and allow for ground effect at altitude, this is relying on safe deploy of landing gear. However, a system like this could also employ a brs because it has no large prop in the center...

@Robert Gomez Small-diameter rotors don't give enough lift in autorotation, and the descent speed will be very high. Even the Osprey V-22, whose rotors are not particularly small,  can't rely in autorotation in case of an emergency... And concerning ground effect, it doesn't exist during autorotational descent, because the wake goes upward from the disk. It's true that, in the last stage of an autorotational descent, the pilot manipulates the collective in order to cushion the landing, thus using the potential energy accumulated in the rotor, but –obviously– that can't be done 'at altitude'...

Oh course, at altitude no auto rotation occurs. Similar to the V-22 it relies on trim and glide ratio. Ground effect can be a factor at near ground level.

@ Robert Gomez   Autorotation can take place at any altitude. For doubters, autogyros fly always in autorotation, and have no trouble at all with altitude...

This is true xxavier, thank you for pointing this out. What I ment to say is, auto rotation can happen at any altitude, however I used the term in the statement to refer to a power off descent in a vertical lift aircraft. Helos with a large diameter propeller do this well as they produce lift simply by the spinning caused by free flowing air, this aircraft relies on a combination of free rotation of propellers, however more from its lifting surfaces like a fixed wing aircraft in a power off configuration.

Not only can it not glide in autorotation, it can't even decelerate in autorotation like a conventional helicopter. Not enough rotor area, it will drop like a rock. It's no big deal if a quadcopter model loses power and hits the ground hard. If it's loaded with people, it's a very big deal. Perhaps they could tie all four rotors together with driveshafts, a la V22, but that kicks the cost way up. Quadcopters may not scale up all that well, when the safety of people overrides the cost benefits.

maybe this idea is interesting for you for further resesarch.
They are developed in my region: Flying taxis:
http://www.businessinsider.com/r-dubai-starts-tests-in-bid-to-become-first-city-with-flying-taxis-2017-9?IR=T

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How does fuel efficiency depend on wing configuration?
Which of the following wing configurations is the most fuel-efficient?

The largest losses on a wing usually occur at the tip, where high pressure air flows around the end from the underside to the top side. This can be minimised by making the wing longer and thinner, or tapering it so the tip is smaller. You can also add winglets to reduce tip losses, but they're tricky to get right.
If you fly near the speed of sound, you get shockwaves that cause a lot of extra drag. You can slightly raise the point where they become a problem by sweeping the wing, and wing sweep also helps to reduce the drag when you are supersonic - but supersonic is never an efficient choice.
Drag is proportional to speed squared, so if you just want to fly for as long as possible, it's most efficient to fly slowly. However if you want to cover distance it's more efficient to fly really high where the air is thin.
You can put all this together to get an airliner, with a long, thin, highly tapered wing (and winglets), flying so high that the air is only just dense enough to support it, and just below the speed that shockwaves cause extra drag.
You ask about wing position, and there is some interference drag where two surfaces meet. It is minimised by making the angle between the two surfaces as large as possible. Thus a mid-wing is slightly more efficient than other designs, but it's a relatively small effect. Wing position is usually determined by the undercarriage design and the pilot's field of view.

Straight wing, look at sail planes ;)
Short story long: the wing configuration itself has not so much of an influence, the position close to zero. It's about the aspect ratio: higher aspect ratio gives you a better glide ratio, thus reducing the power needed. Less power = less fuel consumption (at a given engine efficiency) = better total efficiency. You can also build an inefficient straight wing, let's say one with a quadratic planform.
Problem: large swep back angles (and particularly deltas) compromise aspect ratio geometrically - a 60° delta has given aspect ratio of 2:1 and also high swep back wings bear troubles. You can't build them too long without the position of center of lift becoming too far aft (you might attach them farther up front, but this is limited by the plane's nose... have a look at the B-52's wing attachement position for example). So for a high aspect ratio you need low or no swep back wings.
Yes, such planes are usually rather slow. But, while slightly off topic but related, there is another formula saying twice the speed = four times the energy (or so). This bears the bad news that speed is inherently inefficient anyway...

This is decidedly wrong. The mission speed would dictate which configuration is more aerodynamically efficient. GA speed, straight tapered. High subsonic is dominated by wave drag, so swept. At supersonic, delta. But fuel efficiency is more than aerodynamics...

Jimmy sir, can u explain in better way

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Do vortex generators increase Re of a wing?
Do vortex generators (VGs) increase Re of a wing?
I understand that VGs re-energize the boundary layer on a wing.
I understand it turns laminar flow into attached turbulent flow.
Doesn't that mean the Re increases substantially?
If so, wouldn't that increase it's cl/cd, which means lower drag in the 600-900k Re range?

@Fred, although most people here understand what you meant, it is better to explain the key term you are using. VG is not such a universal abbreviation everyone knows immediately. I took the liberty to edit your text slightly.

You completely misunderstood what the Reynolds number is and what vortex generators do. Re depends on flow speed, viscosity and length, nothing more. And it cannot be dialled up or down, except by changing those parameters.

lol: I guess you're right. I thought wrongly, that Re was also indirectly a relationship between laminar and turbulent flow.

Reynolds number depends on the free-stream speed, the chord length, fluid density and viscosity. So vortex generator does not modify the Reynolds number. However, it trips the flow aft of the device into turbulent flow. Turbulent flow, in fact, has higher drag than a laminar flow (at the same Re). Therefore, on top of the additional skin friction drag introduced by the VGs, the aerodynamic surface would also have higher drag.

Again, the correct answer gets fewer votes. +1

The Reynolds number is a construct that measures the relative magnitude of viscous and inertial forces, and it is defined in a way that is convenient. It is not a measurable physical quantity, so the answer will depend on how you define it.
Note that its expression:
$$
\mathrm{Re} = \frac{\rho u L}{\mu} = \frac{u L}{\nu}
$$
includes a length term $L$ that is arbitrarily chosen as a characteristic length of the problem in question.
This means that one can define a Reynolds number for the free flow over a wing ($L=L_{wing}=c$) and a totally different one for the boundary layer ($L=L_{layer}$). In practice the boundary layer $\mathrm{Re}_{layer}$ and the wing $\mathrm{Re}_{wing}$ will be closely linked as turbulence from the free flow seeps to the boundary layer, but they will not be the same.
For example, the boundary layer near the forward stagnation point will be extremely low and the layer will be laminar, even if the prevailing free flow conditions would indicate turbulence. Of course in such a situation the layer is bound to transition very soon unless the airfoil is specifically designed to prevent this, like the NACA 6-series.
Layer manipulation devices like trip strips and vortex generators muddy the picture further still by altering the conditions in the boundary layer:
A trip strip will force a turbulent transition by means of a local spike in surface roughness, with the intention of using a turbulent boundary layer´s more favourable separation properties.

Vortex generators, on the other hand, mix the flow from the free stream out of the layer into it, increasing its energy, delaying separation. They also increase the $\mathrm{Re}_{layer}$ because they are also a source of roughness  and their modus operandi directly increases the local vorticity.
While it is conceivable that at very low Reynolds a set of vortex generators would operate fully in laminar mode and not transition the layer, this is an academic case, and in the real world you will likely have a turbulent layer downstream of them.

Now, to tackle you question more directly: if you want to increase the effective Reynolds of the airfoil for the purpose of layer attachment, you can obtain some success with vortex generators, and probably even more with a trip strip, but it do not expect either method to take the value over that of the entire wing.

The definition of the Reynolds number does not depend on vorticity. The correct answer to the question is no – there is no way that VGs can influence the Reynolds number, effective or otherwise.

@PeterKämpf I´m afraid your comment that my answer is wrong only had the effect of getting it more upvotes. Could you elaborate? This topic is on the edge of my practical knowledge and I'd appreciate a deeper explanation :)

You say the answer depends. It doesn't. There is only one definition for the Reynolds number and Re grows along the flow path until it reaches what is quoted for airfoils at the trailing edge. Tripping the boundary layer doesn't change Re (and doesn't muddy any waters), and VGs certainly don't increase Re (but boundary layer thickness). Lastly, there is no special effective Reynolds number. Just the Reynolds number. However, the other information in your answer (which is the majority) is correct. I just pointed out the wrong parts.


text:
How can the landing gear of an A320 be lowered in the event of a simultaneous hydraulic and electrical failure?
What is the process for lowering the landing gear of an A320 when the hydraulic and electrical systems are not operational?  What happens to the aircraft when you lower the landing gear with inoperative hydraulic and electrical systems?

What do you want to know? How the gravity extension works? What linkages are involved?

Yes. How does the gravity extension works and what happens to the aircraft when you lower the gear when hydraulic and electrical system is not operational and what linkage are involve.

Related

The A320 has an emergency gear extension handle to deploy the landing gear by gravity in case of electrical, hydraulic, or mechanical failure. To force the landing gear to extend, one of the crew must pull the emergency gear extension handle up, then turn it clockwise three turns. The cutoff valve then shuts down hydraulics to the landing gear system and depressurizes it. This allows the landing gear to fall free to the down lock position by gravity.
Source: a320dp.com


text:
What happens when you add a vortex generator to high camber LAMINAR wing?
What happens when you add a vortex generator (not trip strip) to a high lift or laminar wing, operating say at 500k to 1,000k Re?
I understand vortex generators add a little less lift than a slat on a classic naca wing, about 0.6cL.  I understand that vortex generators are very different than trip strips, as they actually  add energy to the boundary layer.  I understand that both vortex generators and trip strips change the flow from laminar to turbulent, but trip strips don't add cl like a vortex generator does.
So ... what happens when you add a vg to a high lift wing like the selig 1233 or the eppler or Wortmann series of reflex or non-reflex laminar wings.
Do vortex generators still add about 0.6Cl or is it actually detrimental as these wings are designed to operate in laminar flow, not turbulent flow?

I did a lot of research on VGs some years ago when scratch building bits of a homebuilt project that never went anywhere.  On thick wings with blunt leading edges,  the Cl improvement is somewhat less than when you add them to a wing with sharp leading edges.  Anyway, my plan was to build a Pegazair, which has automatic slats, without the slats and just drooping the LE and adding radius to the Peg's NACA 2415, and adding an array of VGs.  I only took credit for a VG Cl improvement of about .4, and based on that I calculated the minimum flying speed as only about 4 mph faster than with slats.

I have them on my 1973 Cessna 177B, the plane has the same control feel well below the stall speed and I can land in a shorter distance. The plane's "wing airfoil was obtained by fairing the front half of the cambered NACA 2415 shape onto the previous NACA 65A015 airfoil. This eliminated the laminar flow benefit of the 65-series airfoil (whose low-drag "bucket" at a low angle of attack had not been properly exploited in actual use), but gave a better lift-to-drag ratio at high angles of attack (i.e. during the climb), and softened the break at the stall." www.skytamer.com/Cessna_177B.html

@CrossRoads unrelated question: does your Cardinal's nose oleo make little thumping/tapping noises while taxiing?  My '67 did that and I was told it was because it was a floating piston oleo and the thump was the piston.

No mine doesn't. Are you a member of Cardinal Flyers Online (CFO)?  https://www.cardinalflyers.com  There are several articles about fixing things like that. Sometimes it can be adding a simple shim to the top of the strut, other times the strut needs a rebuild with new O-rings. Join up, do some reading, ask a question via the digest that goes out nearly daily.

Thanks for the answers. I'm a total fan of Vg's too, Put them on my Zenith Ch601.  I fiqure, they'll probably save my life one day- lower landing speed, delayed stall, but the answers above are based on the Naca2415 which is not a LAMINAR  foil. Still wondering what would happen on a laminar high camber foil, like a Selling 1233.


text:
Does a high-pitching-moment symetrical airfoil exist?
I'm looking for a high-pitching-moment symmetrical airfoil.
I understand the Zenith 701 uses a reverse NACA4412 as it has a high pitching moment. This allows for a smaller horizontal tail.
What about a symmetrical high pitching airfoil for the vertical tail?
Wouldn't this reduce the size of the vertical tail?
Does a symmetrical airfoil with a pitching moment exist?
How about one with a high pitching moment?

By definition, the pitching moment of a symmetrical airfoil is zero.  Cambered ones tend to pitch up as AOA is increased.  These descriptions generally pertain to wings.  Do you mean pitch authority?

High Cm would be a better description. Just wondering if there is an airfoil series with a large Cm. Isn't this a free lunch? Higher Cm means smaller tail volume, no?

Cm = Pitching Moment/(Dynamic Pressure q ×Area×chord) no luck there, plus draggy.  Not a big fan of tiny tails.  You need to control that wing.  Airfoil tails can stall.  Larger flat plates as seen on the Piper Cub are tried and true, especially at cruise.  Just doing their job holding pitch.  You trade off needed pitch control (including elevator) vs drag in all cases.

What you probably look for is a symmetrical tail airfoil which produces high control moments. Since those moments are the product of force and lever arm, placing those control surfaces far from the center of gravity produces high moments.
A symmetrical airfoil will have zero pitching moment around its quarter chord point in attached flow. This is understood as zero pitching moment, since the pitching moment of an isolated surface is per convention defined around its quarter chord point. For a thorough explanation please consult this answer.
If, however, you choose a different reference point for your moment, even the symmetrical airfoil can produce a pitching moment. That effect is used in tail surfaces and for a given geometry, a higher maximum lift coefficient will produce a higher maximum moment. Traditionally, the NACA four-digit range of airfoils starting with a double-zero have been used for tail surfaces, but more modern designs will produce a higher maximum lift and lower minimum drag. A good choice would be the Wortmann FX 71 range of airfoils, like the FX 71-120 or the FX 71-L-150 which can often be found on gliders.

I'm not aware why you need a large pitching moment, but if you want to have a strong self centering tendency, and more authority when displaced, design it with a trailing edge anti-servo tab, like this.

The anti-servo tab would increase the rudder effectiveness and the control hinge moments, but I don't see how it would increase the directional stability?

If the plane yaws, the rudder will try to trail, but as soon as it starts to displace from neutral aligning itself with the airflow, the anti-servo tab is moved in the same direction, which drives the surface back to anti-servo-neutral, which is also rudder neutral. Stabilator all-flying tails depend on an anti-servo tab for static pitch stability.  Otherwise, you'd have to physically hold the control all the time. Some gliders have all flying tails w/o anti-servo tabs, but they get what static stability they have from the trim bungee spring, and they are stick fixed all the time anyway.

I see you're talking about a reversible control. And in the context of the ultralight it makes sense.


text:
What does Lockheed have to do to mark the end of the F-35's SDD?
What does Lockheed Martin have to do to mark the end, or close out, the System Development and Demonstration phase of the F-35?

This is really a contracting question.  LM has to satisfy the SDD exit criteria that's defined in their contract with the F-35 program office.

The best indication of what remains to be done is given by the annual US DOT&E reports. The latest edition is here: http://www.dote.osd.mil/pub/reports/FY2017/pdf/dod/2017f35jsf.pdf I could summarise it to produce a swanky answer, but to be honest, that's quite an effort.

Are you asking about the A, B, or C model F-35?

I was under the assumption that the end of SDD would be a program event for all three simultaneously. Will each variant close out SDD separately?

The clearest explanation I have found, comes from the Congressional Research Service (www.crs.gov), from RL30563.pdf:

The F-35 is currently in low-rate initial production, with 280
  aircraft delivered as of April 2018. At least 250 of those were in
  U.S. service. Four to five aircraft are currently delivered each
  month, with the production rate scheduled to increase to 120 per year
  by 2019. In keeping with the acquisition plan that overlapped
  development and production (known as “concurrency”), the F-35 was also
  in system development and demonstration (SDD), with testing and
  software development ongoing, from October 2001 until April 11, 2018.
  The SDD phase will formally continue until the end of Initial
  Operational Test and Evaluation, when a “Milestone C” full-rate
  production decision will be made.

A copy of the full PDF is available here:
https://fas.org/sgp/crs/weapons/RL30563.pdf
We know from other sources that Initial Operational Test and Evaluation (IOT&E), will formally begin later this year, 2018.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the climb/descent rate that triggers the TCAS up/down arrow?
If Aircraft One is maintaining a low rate of climb/ descent, is there a minimum descent or climb rate threshold below which the up/down arrow will not show up on the TCAS of the Aircraft Two?
I have heard figures of 400 fpm and 500 fpm.
Is there an industry standard, do standards depend on aircraft performance / weight class?

Got the answer. It's 500 fpm.  Below this vertical speed of the target aircraft, V/S arrow is not displayed.

It is also manufacture dependent. For example, I just read in an aviation article that some airplane glass cockpits such as the Cyrus use a 6 second future event timeline and other aircraft such as Boeing use 10 seconds. Emergency and warning criteria does not seem to be standardize or regulated.

@jwzumwalt it is NOT manufacturer dependant.The TCAS II implementation is very mich standardised and regulated. The TCAS II algorithm logic is prescribed in RTCA DO185B / EUROCAE ED14 Minimum Operational Performance Specifications for TCAS II.

The RTCA DO185B does not necessarily provide limits. It uses terms like, ..." If the intruder gets within 15 to 35s..." and "nominally between 1,500 and 2,000 fpm.", but these are suggestions and not regulations. Referencing a document such as RTCA DO185B without a
applicable quote is meaningless to a discussion such as this. RTCA DO185B sets a standard for the audio/visual WARNING MESSAGES  but does not set a criteria for what triggers the warning.

@jwzumwalt the TCAS II MOPS are not suggestions, and also they contain much more than warning messages, it is full of detailed technical requirements and  pseudocode for implementation. If you can get  a copy, I recommend reading it, it's an interesting read. See my answer for the relevant quote.

The Minimum Operational Performance Specifications (MOPS) for TCAS II (RTCA DO185B / EUROCAE ED143) specify the requirements for TCAS II.
It states that the arrow must appear when the vertical speed of the intruder is 500 feet per minute or more:

2.2.6.1.2.1.6
Intruder Vertical Speed Arrow
A vertical arrow shall be placed to the immediate right of the traffic symbol if the vertical
  speed of the intruder (as determined by the TCAS tracker) is equal to
  or greater than 500 fpm, with the arrow pointing up for climbing
  traffic and down for descending traffic. The color of the arrow shall
  be the same as the traffic symbol.

Source: RTCA DO185B / EUROCAE ED14 Minimum Operational Performance Specifications for TCAS II, volume I.
Whilst the MOPS are not regulations themselves, the  aviation authorities put them into regulations such that TCAS II needs to be according to the MOPS.
In the US, the FAA has published Advisory_Circular AC 20-151A which states:

The FAA ACO approving the initial
   installation of the TCAS II equipment must verify that the TCAS II processor design does not differ from the criteria
  specified in RTCA/DO-185A or
  RTCA/DO-185B as applicable.

This means it is not up to the manufacturer to decide when to show the arrow symbol. It is required by the MOPS, so it has to be designed that way.


text:
Do airlines have freezers?
In the latest twist to the case of the dead rabbit, an anonymous "airport worker" has told a reporter that it was killed by leaving it in a  freezer locker, contradicting the accounts of United executives who have claimed the rabbit was well treated. United also hastily incinerated the body without the owner's permission, preventing any kind of autopsy from occurring. This all happened apparently at Chicago O'Hare.
My question is why an airline would have a freezer at all, and why would they be putting luggage or pets into said freezers. I mean when I make connections I kind of assume they are not putting my bags in a freezer. Do airlines have freezers?

You are using "The Sun" as a reference?  Do  not believe a single word that "newspaper" publishes.  Even then, you haven't read it correctly - I quote - "He said: “We know from the inventory that that the rabbit was alive when it reached the airport. So it happened in the warehouse."

@Simon I understand that. That does not change my question. My question reads "airline" not "aircraft". An "airline" includes their ground operations.

I would point out that the article lists the "freezer" temperature as 0-2°C, which is not the temperature for a freezer, but a chiller. I think the article (and the source, if accurately quoted—big if) has incorrectly referenced the equipment as a freezer for dramatic effect.

A rabbit at O'Hare? Was the article dated 1 April?

@FreeMan: If you hadn't said it, I was about to! ;-)

Parcel services use temperature regulated containers for shipping various products. Pharmaceuticals come to mind. Human organs are normally packed as independent packages, but are sometimes placed in thermally regulated containers. Substantial amounts of produce are shipped via air transport.
Hubs have temperature controlled areas for cargo. It can include produce, pharmaceuticals, etc.  Even parcel "stations" for ground delivery services have them when they have customer needs.

and sometimes things that need to be kept very cold are shipped in a thermos filled with liquid nitrogen. Not joking, it's done :)

It is done but dry ice is preferred.  LN is more hazardous in the event of a container breech.

Cargo on airliners sometimes have unique requirements.  For example, deceased human bodies are commonly carried on passenger planes. The bodies tend to leak fluids, and can be heat sensitive. They are chilled prior to loading, and in high ambient temperatures are off loaded should there be substantial delays. At altitude, bleed air is used to manage heat to the cargo area, and cadavers in the cargo area will bias the temperature colder by many operations. Similarly, live pets in the cargo may bias the temperatures upwards. Company dependent.

My dad worked for a company (as CEO) shipping bull semen and cattle embryos around the world for exchange of genetic material between breeding firms. They did it just like that, a steel thermos filled with liquid nitrogen, carried in the cabin luggage of a courier (with proper documentation of course). Speed is of essence during such shipments, even having it gate checked can cause delays long enough that the material degrades. LN lasts long enough for the purpose, and is cold enough, dry ice isn't.

Mind we're talking life genetic material here, not dead.

I have seen pallets of Dewars with bovine semen on them in a Fedex hub in Memphis. There was a styrofoam top and strapping over it to help keep the Dewars upright. Dewars will keep material cold for a month or longer. I think the point is that carriers and cargo operations regularly have the need to transport things which may require some level of environmental support, whether they are self-contained or require some accommodation in the cargo and hub environment.

Fedex staff at Memphis have told me stories about transport of ocra (killer whales) and giraffes. Apparently the giraffes like to eat cabling in the upper fuselage during flight.

wouldn't surprise me. That cabling probably looks like sapling branches to them, which make up a good deal of their diet in nature. Time for those blankets to cover the ceiling :)

I'm not aware of any American carriers that have freezers onboard that are accessible to the crew during flight.  For catering purposes, dry ice has been the accepted means of keeping food chilled until preparation and serving.  It's primary use was keeping food at cold temperatures while transporting to the aircraft and while on the ground.  It wasn't needed once the food was boarded and usually was removed by the flight attendant when they signed off on catering. Once chilled, the food containers remained that way for the short time until service was started.  The exception to this would be if you had a long haul flight with two services then you would keep dry ice the the food carriers for the second service. Refrigerators and freezers for cargo transportation purposes are a separate subject.

"Refrigerators and freezers for cargo transportation purposes are a separate subject." But it's the subject this question is about...


text:
Why would an approach be Cat A NA?
I was looking at the RNAV (RNP) 6 approach into Gunnison (KGUC) and noticed something quite strange:

If the image isn't clear enough -- the approach is Not Authorized for approach category A aircraft (which of course includes every last helicopter on the planet).  WHY would this be so?  Is there some operational or technical reason a slow-moving category A aircraft or helicopter can't fly this approach, even though their avionics might be perfectly capable of doing so? Or is this some sort of charting error I should bug the FAA about?

It's not a charting error, or at least the TERPS form was created with category A NA: https://www.faa.gov/air_traffic/flight_info/aeronav/procedures/application/?event=procedure.results&tab=ndbr&nasrId=GUC#searchResultsTop

@NathanG -- good find

Maybe there is a separate procedure designed for cat A aircraft?

The aircraft category refers to the flown approach speed (Vref or 1.3Vs0). A pilot may always choose to use a higher category (e.g., a pilot flying a normally Category A aircraft may use the Category B [or C, or any other higher speed] approach minima. @J.Hougaard, there are other approaches, including an ILS to runway 6, which do not have NA for Category A; the RNAV (RNP) 6 approach has the lowest minima. Regardless of category, the flight crew requires authorization to fly the approach.

@ammPilot -- is being able to use higher approach Category minima still true for helicopters?

RNP is 'Required Navigation Performance'  it is highly unlikely that a small aircraft, that has a Vref or 1.3Vs0 of less that 90 kts will have the RNP equipment. If for some bizarre reason they do, they can use the Category B.

@JScarry -- I'm thinking of STOL turboprops and helicopters here, mainly (those would be the most likely critters to be RNP equipped and Cat A)

@UnrecognizedFallingObject, following is from the US Aeronautical Information Manual (AIM), section 5-4-7.a:
"Helicopters may use Category A minima. If it is necessary to operate at a speed in excess of the upper limit of the speed range for  an  aircraft’s  category,  the  minimums  for  the higher  category  must  be  used."
Note, though, that this still doesn't explain the lack of Cat A minima when Cat B and Cat C are provided. (NA for Cat D, I understand: nothing Cat D, and no one flying at Cat D speeds on the approach, may fly this approach.)

@ammPilot -- alright, I think it's clear now that helos use whatever minima for the speed they're flying at, just like everybody else.

Could it be possible that with cat a speeds, flying a radius-to-fix segment with a constant turn would result in too low bank angles?

@Sami -- I'm not sure why having to fly a shallow turn would be a problem for a slow plane?

BTW -- I just sent a brief email (with the picture from this post linked) to the  AeroNav Products folks (using the contact info found here).  We'll see if they get back to me within 7 days!

@UnrecognizedFallingObject I don't know either. It was just an idea. I'm not sure if autopilots for example are capable of maintaining a turn with 1-2 degrees bank.

@unrecognizedFallingObject I can't wait to compare notes. I used the FAA's online "Aeronautical Data Inquiry" form, instead of email, but it will be interested to see what we each receive and how closely they match up.

It looks like the AeroNav Services folks didn't meet Ralph's deadline for the bounty...I'll still keep an eye on my email and respond with an answer if they reply, though

They did reply (1 day after the bounty deadline expired, but Jimmy takes the cake anyway :) and verified that Jimmy is correct! I'm still not 100% sure what the design logic behind that chunk of 8260.58A is, but I'm seeing evidence that light plane pilots may have to start looking out for Cat A minima adjustments or even NAs on other approaches where they may not expect to find them!

Original answer: The speed of Category A aircraft is too low to execute the missed approach.  For this particular airport, it's basically a hole in the ground with steep mountains on all sides. To go missed, you gotta get up in a hurry, and the TERPS data probably indicates a minimum speed is needed.

Edit:  Despite the downvotes and comments stating that my answer was "unlikely", I'm sticking to my guns.
To make sure that I knew what I was talking about, I contacted the FAA's Flight Procedures Standards Branch in OK City and eventually was put in contact with someone who was able to pull up the documentation on this approach.
Listed in the comments of the document he was looking at was this: "Ref 8260.52 CAT A -- final approach speed is too slow for RNP missed approach segment length for obstacle penetration."
Translation: whoever put this approach together did the math that is required, and the formula result was that speeds needed for obstacle clearance are not sufficient when using CAT A speeds.
Note 1: some math and formulas required to build an IAP missed approach procedure can be found in Chapter 4 of 8260.52. (https://www.faa.gov/documentLibrary/media/Order/ND/8260_52.pdf)
Note 2: 8260.52 has since been superseded by 8260.58, which itself has undergone an update and the most current document is 8260.58A.

Gory technical details (or some insight as to why this oddity is the way it is):
The thing that makes this approach special is that not only is it a RNP AR approach, the missed approach requires abnormally precise navigation as well (i.e. RNP values <1.0).  The length of this span of the missed approach (from the MAP to TIPOC) is bounded by the results of formula 4-3-8 in 8260.58A, which is dependent on the true airspeed of the slowest aircraft of the approach.
DMASRNP is the max distance allowed for a given true airspeed (category) to return to RNP 1.0 on missed approach. The longer the distance to return to RNP 1.0, the faster the plane needs to be.
In the case of the KGUC RNAV (RNP) 6 -- the MAP to TIPOC span is 6.7NM along track, which is longer than the formula's results no matter which of the three possible RNPs are applied if you are using a category A airspeed (these results are applied to the whole span as per figure 4-3-5).
Why this formula is needed, and applied to the whole span for that matter, is a mystery, though -- 8260.58A gives no design rationale for this limitation, or why it is pegged to airspeed.  A rationale is that navigation errors are cumulative though -- the slower you're flying, the more time you have to drift off course before you make it to the next "gate" where a higher RNP prevails.
Other approaches (even in similar terrain and with seemingly similar conditions charted) may not have this peculiar limitation, though -- the KRIL RNAV (RNP) Z 26 approach, for instance, also requires RNP < 1.0 on the missed approach, but for a shorter distance -- a mere 3.5NM from the MAP to TEROE, whereas the limit distance for Category A given the RNPs used in the missed approach there is 4NM.
Moral of the story: pay attention to your approach plates!

...I don't see how having a higher airspeed on approach would help you with your sustained missed approach climb rate (especially if you're in a helo, where you can climb quite steeply at low airspeeds if you so desire AIUI)

If you have a STOL turboprop that can fly slow & climb like a rocket, then the missed approach gradient should be achievable.  And if the gradient is beyond the aircraft's capabilities, then THAT fact would prohibit the aircraft from flying the approach, not its Cat A approach speed. I found nothing in TERPS that gives a maximum assumed climb capability for Cat A aircraft, so this answer seems to lack credibility.

@ralphJ, I added sources and references to hopefully get my credibility back. :)  These were provided by an FAA employee who was looking at the original source document for this approach.

Also.. I was looking at NathanG's source link (https://www.faa.gov/air_traffic/flight_info/aeronav/procedures/application/?event=procedure.results&tab=ndbr&nasrId=GUC#searchResultsTop), and I could not find what the FAA employee was referring to, but I'm not positive we were looking at the same document.

The two docs (linked + updated) are COOL documents (in a really, really nerdy sort of way!), but I'm not finding any formula where LOW airspeed creates an issue. High speed, sure -- and Cat D+E are NA in that approach -- not surprising. But if you meet the GRADIENT, why does your (low) speed matter? High speed can spit you out wide on a turning (RF) segment, but low speed doesn't drop you inside -- you're just at a lower bank angle to maintain track. So I guess I'm still not seeing it, but I'm willing to be shown what I missed where a low speed disqualifies during missed approach.

I'm not sure what the FAA bloke was looking at, but the explanation still isn't making sense to me (see the KRIL RNAV (RNP) Z RWY 26 for a comparison point)

In addition to the verbal confirmation from the FAA (that my answer was spot on, regardless if we understand the logic or not), I've also put in a formal "Aeronautical Data Inquiry" about this chart. Not sure how long that takes to get a response, but I'll post the information I receive when I do.

I just heard back from the AeroNav folks, and lo and behold, your answer is correct!

Quoting the email I got back from the FAA: "CATEGORY A NA: CAT A FAS AIRSPEED TOO SLOW TO SUPPORT DMASRNP DISTANCE NEEDED MISSED APPROACH OBSTRUCTIONS. MAXIMUM DISTANCE MISSED APPROACH FOR RNP (DMASRNP) CALCULATED lAW AFS-400 MEMO "CORRECTION TO ORDER 8260.52 , U.S. STANDARD FOR RNP SAAAR", DATED MAR 14, 2008."

Reviewing 8260.58A, what they're referring to is the maximum length of the reduced-RNP missed approach legs on the procedure, which is computed using the missed approach KTAS for the slowest CAT that uses the approach (from formula 1-2-7, using the DA for the altitude and final KIAS for that CAT) plugged into formula 4-3-8 along with the limiting RNP 0.3 from the MAP to POSLY yields 2.96NM, which while longer than that limiting leg, is shorter than the sum of the RNP < 1.0 missed approach legs (see figure 4-3-5).

Likewise, if you use the RNP 0.44 from the leg from POSLY to JENRO or the RNP 0.51 for the leg from JENRO to TIPOC, you still get a value that limits the sum of the length of the RNP < 1.0 missed approach legs to shorter than the total of 6.7NM you have to fly the missed approach with RNP < 1.0.  I suspect this is because of error accumulations being proportional to time, not distance, and a slow Cat A aircraft has too many chances to drift off course and smack into a rock if they tried to fly the approach...or is there some other explanation for the formula 4-3-8 limit?

Well, I guess that answers two questions: Which response will be faster?Your email response was faster than my online "data inquiry" form response (I still haven't heard back). And that pasted text is pretty much what the FAA guy on the other end of the phone told me, and which I then mangled up in translation when attempting to post that answer here. Thanks for posting what they sent.

Doing a similar analysis (for comparison purposes) for the KRIL RNAV (RNP) Z 26 approach (with a DA a couple thousand feet lower that shaves a few knots off the TAS of the aircraft, but with the limiting leg being the RNP 0.4 SIYOP-PUVOY, yields a value of 4NM, and you get to RNP 1.0 at TEROE after flying 3.5NM of that missed approach, overall.

@unrecognizedFallingObject , I got the following response back on my FAA query: “Category A NA because CAT A FAS airspeed too slow to support DMASRNP distance needed to avoid missed approach obstructions.”  (DMASRNP = The maximum Distance that the <1.0 RNP value may be extended into the missed approach surface (MAS)).

@Jimmy -- yeah, that's pretty much spot on to what I got back -- do you want to edit my commentary into your answer, would you rather I do that, or would you rather I post my commentary/analysis as an answer unto itself?

@unrecognizedFallingObject, I think it makes sense to roll it into my answer. Feel free to do the edits. After looking at the actual process for this answer (and the Seattle one), I realize I have much less confidence in my understanding of the whole process.


text:
Is a high alpha sideslip possible and safe?
Is a high alpha sideslip possible and safe?
We use a sideslip to increase drag when trying to reduce altitude before landing on a short runway, etc., which is crossing your rudder and ailerons, to produce an angle to the intended direction, usually in level flight.
Is it possible to do a sideslip at an AOA of say 15 deg for even more drag, assuming stall angle is at 30 deg?

Let's assume that to  first approximation, a given rudder deflection (say, full deflection) creates a given slip angle of the fuselage.  So for a given rudder deflection, don't you generate a lot more drag (and also sideforce- thus requiring a steeper bank angle if you wish to maintain a linear flight path) if you are flying at a HIGH airspeed than at a LOW airspeed?   So, there may be a wrong premise imbedded in this question.

What kind of aircraft are you flying which stalls at 30°? Convair F-106? MiG-21?

You would be risking your life.  Cross controlled approaches should only be done well above stall speed and only after testing at a higher altitude.  Increasing AOA will slow your plane down.  If you recognize being high early in the pattern, you might be better off going ABOVE Vbg and slip, which makes the plane glide much less efficiently.  Spoilers act in a similar manner.
Do a proper approach and use your flaps/spoilers.  If it is allowed, forward slip to increase rate of descent at your preferred approach speed.  If you can't make the runway GO AROUND.  Changing speeds on short final will make judging the landing much more difficult.
In an emergency landing it is far better to over shoot the field and roll into fence, trees, etc at 10-20 mph than to risk losing control of the plane.
As a side note, only deltas come close to 30 degrees AOA before stalling, and these tend to drop like a rock when slipped if their vortex lift is sufficiently disturbed.  Slats and flaps will significantly increase stall AOA, but at the expense of much more drag.
Generally a bad idea.

Zenith 701 pilots do short takeoff or landings ( STOL) at 30 deg AOA all the time, I'm told. Just not sideslipped I guess.

@Fred from my own experience in a 172 the airflow can get pretty crazy when slipping with everything "hanging out".  So I would check the POH and talk with pilots experienced in operating that particular type of plane.  It may be possible, I just wouldn't do it near stall.

Flaps actually decrease the stall AoA a bit (but still increase the maximum lift). Slats increase the stall AoA, which is why they are always extended before flaps to compensate for the reduction due to flaps.

@Jan Hudec that is true, relative to the AOA sensor (on the fuselage). Both create a higher lifting cambered wing, which allows lower AOA, or airspeed, for the same lift.

Let's assume that to  first approximation, a given rudder deflection (say, full deflection) creates a given slip angle of the fuselage.  So for a given rudder deflection, don't you generate a lot more drag (and also sideforce- thus requiring a steeper bank angle if you wish to maintain a linear flight path) if you are flying at a HIGH airspeed than at a LOW airspeed?   So, there may be a wrong premise imbedded in this question.

That's a good point. You can get steeper descent on the back side of the power curve with increasing AOA; but flaring on the back-side would be a whole different story...

That is exactly right +1.

Yes it's safe as long as you know what you are doing and there is adequate alpha margin while slipping, and you are adept at managing the energy of the very high sink rate that results.  If you were approaching in something like a 701 at, say, 35 mph, and you wanted to slip to increase the descent rate, you could do so but it would be prudent to reduce alpha somewhat and add energy by speeding up a few mph while in the slip.  So you would do it like any other airplane, kick in the slip and lower the nose a bit at the same time, maintaining ample margin above stall.  So if the stalling AOA is 25-30 deg and you are at 15 at your normal STOL approach speed, not a problem.
The main thing is, in a STOL airplane with very high drag in the landing configuration, you have to make sure you have energy to arrest the sink rate close to the ground by having excess inertia energy and being ready and aggressive with power to control the last bit before touchdown.  The wing may not stall until it's at 30 deg, but when it's getting up there the drag is massive and the increase in lift with extra pitch declines and you can find yourself dropping out and landing hard, even though you are not actually stalled.


text:
Why do gliders have bungee cords in the control systems and what do they do? Are they on all control surfaces? What about ultralights?
Why do gliders have bungee cords in the control systems and what do they do?
Are they on all control surfaces?
Why don't ultralights have them?

Could you add some more information about what you mean by bungee? A quick Google search finds Bungee Launch and Bungee Springs for trimming.

You should include a more accurate description of the bungee (maybe a picture?)

The bungee is just a bidirectional spring that tends to hold the elevator at position x, and if you move the stick you are stretching the spring in one direction or the other.   They are used in the elevator control circuit to provide an adjustable centering force for trim purposes, and on gliders with all flying tails, a measure of stick free static stability (you don't get a lot - many gliders are mildly divergent in pitch if you aren't holding the stick, but this isn't a big deal since you are almost never flying hands off).  The glider's pitch trim control adjusts the "at rest" or relaxed position of the spring so it will tend to fix the elevator at this position or that unless overcome by a control input.
Normally you would have a trim tab on an elevator/stabilizer tail or a trim/anti-servo tab on an all flying tail, as regular airplanes do to provide speed-seeking trim and strong stick-free stability, but gliders operate with much lower loading and forces on the tail and can get away with a bungee spring that acts like a little man under the floor pushing on the stick, this way or that to try to hold it at a given position, so you don't have to.  This does away with all the linkage, tab surface etc, which is a big deal when you are shooting for a 40:1 or more L/D.
You can put bungees in any control circuit if you want to be able to trim it with an adjustable centering force, or want to increase the forces.  Some light aircraft use bungee springs in the rudder control circuit to provide rudder trim.  The spring just replaces your foot by exerting a spring force so you don't have to.  The downside is the spring loads are present when you move the controls out of the trimmed position, adding to the perceived rudder forces when maneuvering.
I'm pretty sure there are ultralights out there that use elevator bungees for trim purposes instead of a tab.  However, driving the surface directly aerodynamically using a tab on the surface provides a more sensitive and robust solution and if I was designing an ultralight, where drag isn't that big a consideration, I'd use a tab for the elevator, and perhaps a bungee in the rudder and aileron circuits if I wanted rudder and roll trim.

JohnK is right, they are helping to trim the aircraft. But there is more to them.
Ideally, if you want to modify the hinge moment of a control surface, you would add a Flettner tab (in order to decrease control forces, like done on the SB-10 rudder) or an Anti-Flettner tab (in order to increase them, like on the Diamond DA20 Katana elevator.). No bungee or spring needed, and works equally well over the full speed range.
But that is the point – the correcting action of a tab grows with airspeed. On the other hand, if you add a spring in the control linkage, its force only depends on the stick position. Speed does not influence its contribution to control forces.
Did you ever notice that those bungees or springs are coupled with a cambered trailing edge? On elevators only? They are – always! And that is the point to their existence: That camber also will add a hinge moment contribution, and one which grows proportionally to dynamic pressure. Now you have two elements which add their own hinge moment contribution, one constant and one proportional to dynamic pressure. And that is key to their function: Together they increase stick travel and force over speed.
The trailing edge camber will try to push the elevator trailing edge up, which decreases the elevator's deflection angle. The bungee or spring pushes against that so the elevator stays at the desired angle. If the aircraft speeds up, the camber contribution now overpowers the bungee or spring and the elevator moves to pull the aircraft's nose up. When the aircraft slows down, now it is the spring which overpowers the trailing edge camber and the elevator is pushed trailing edge down in order to lower the aircraft's nose.

Stick force variation over speed decreases with speed, without that trick the stick force response to speed changes would be too small at high speed. By adding the camber-spring combination, gliders achieve proper speed stability over their whole flight speed range.

Why do gliders have bungee cords in the control systems and what do they do?

Coupled with trailing edge camber, bungees or springs in the elevator control linkage help to increase the speed stability of aircraft.

Are they on all control surfaces?

No, elevators only.

Why don't ultralights have them?

Some do, but most don't need them due to their limited speed range.

So if I understand correctly, this mechanism compensates for the decrease in aerodynamic pitch stability at high speeds and thus allows having less of it and the associated trim drag, right?

@JanHudec: Less of it at low speed, yes. It is mainly for comfort, though.

I've been flying a Junior lately, which has a moulded in tab extension on each elevator with a bit of down deflection to drive the elevator up and preload the bungee.  It works quite well and has very effective speed seeking when moving the trim control.  Being neutral in roll, I can bank into a thermal, pull the trim knob full aft to command minimum stick free speed, around 40kt, and cross my arms, and it will hold the turn quite nicely for a couple times around until the bumps upset things and it slowly starts diverging, while holding the turn.

If you want to custom tailor the stick force relative to position (and indirectly, speed) you can replace the bungee acting directly on the control circuit with a profile cam with a spring loaded roller that wants to nest in the middle of the cam profile.  When you move the stick you move the cam and force the roller up the profile in either direction, providing a return to center force.  You can very the cam profile to make the force buildup non-linear if you want.  Make the roller arm movable to allow the neutral "nest" position to be changed, and there's your pitch trim.


text:
What is the minimum distance allowed between two parallel runways?
Is there an international regulation for this matter or does it differ from one country to another?

In cairo International airport the distance between RWY 05L and 05R is 4250m and we just had instructions issued today that when having landing traffic 6nm final RWY 05R no departure permitted on RWY 05L.untill the arrival traffic is on ground even there is successive arrivals.... So the departure will hold short the other RWY undefinite time

In the United States, the FAA has published Advisory Circular 150/5300-13A, Airport Design , which includes standards and recommendations for airport design, including parallel runway separation.
In summary,

"For simultaneous landings and takeoffs using VFR, the minimum
separation between centerlines of parallel runways is 700 feet (213
m)."
For simultaneous IFR operations, "Dual simultaneous precision
instrument approaches are normally approved on parallel runway
centerline separation of 4,300 feet (1311 m). On a case-by-case
basis, the FAA will consider proposals utilizing separations down to
a minimum of 3,000 feet (914 m) where a 4,300 foot (1311 m)
separation is impractical. This reduction of separation requires
special high update radar, monitoring equipment, etc." Note:
Simultaneous Offset Instrument Approaches allow as little as 750 ft
between runway centerlines.

International regulations do differ. ICAO have their own standards. Here is an example of a document which prescribes airport standards compliant to ICAO Annex 14.

And by a not so amazing coincidence, it just so happens that the parallel runways at SFO are 750 ft centerline to centerline. I've heard that the FAA picked this value for SOIA operations for SFO, but don't have a good cite.

That's true! And 750 ft feels darn close when wingtip-to-wingtip on the 28s on a visual. It's probably no coincidence that we use SFO to practice Simultaneous Close Parallel breakouts (in the sim).

The two runways at KBFI seem to have centerlines slightly under 500 ft apart.

Yes, you'll find closer spacing than the FAA recommendations for existing airports.

@abelenky But are they allowed to be used simultaneously? You could presumably have two runways ten feet apart if you didn't want to use them at the same time.

And to note the exceptions to every rule, observe the (unofficial) grass strip to the right of 32 at 5B2 (https://www.google.com/maps/place/Saratoga+County+Airport/@43.0482428,-73.8593018,773m/data=!3m1!1e3!4m2!3m1!1s0x89de47726b02c641:0x2180f739f376c666).  When things get busy, we frequently use this runway to simultaneously land multiple gliders, and runway 32 is only 100 feet wide.

A little late to the party, but Gatwick is an example of two runways that are too close together: Gatwick has two runways (05L/26R and 05R/26L) but only the former is used, as the latter is too close to it. 05R/26L is only used if 05L/26R can't be used (maintenance etc)

ICAO standards and recommended practices are not the ones linked in the answer. ICAO Annex 14 and ICAO DOC 9157, both of which directly relate to this question, can be found (bought) here: http://store1.icao.int

Thanks for the input @molgar. I've revised my answer to specify that this document is only an example of a compliant one.

Those are guidelines and not regulation (you never answered the question). There are cases of runways right next to each other, far less than 700 feet.

Based on ICAO doc9157 Aerodrome Design Manuel Part1 Runway, the minimum distance between centre lines of 2 parallel( or near-parallel ) runways is
For Visual Meteorological Conditions (VMC) operation:

210m for category 3( runway length=1200m-1800m ) and category 4(
  runway length>1800m )
150m for category 2( runway length=800m-1200m )
120m for category 1( runway length<800m )

And for Instrument flight Condition (IMC) operation:

1035m for independent parallel approaches ( radar separation minima
  between aircraft and adjacent centre line is not prescribed)
915m for independent parallel approaches ( radar separation minima
  between aircraft and adjacent centre line is prescribed)
760m for independent parallel departures or Segregated parallel
  operations (one exclusive for departure and another for landing)
PS: It can be reduced by 30m for each 150m arrival runways is
  staggered toward arrival aircraft down to 300m between centre line of
  two runways; and vice versa (but no maximum)

When two runways are closer than the regulations above, only one can be "operated" at the same time (eg: LGW). Even though only one of the parallel runway would be operated at the same time, the separation of the runway have to follow the "Runway strip rule", which is

75m each side from the centre lines for category 3/4
40m each side for category 2
30m each side for category 1

Therefore minimum distance of two parallel runways is the sum of the strips,ie: minimum distance of two parallel 4E/F runways is 75+75=150m from their centre lines.
The above is global standard which represents the minimum requirement of runway , other aviation authorities such as FAA in US may have stricter standard.

Some countries may have other rules but it is a minimum.

The distance between the runways is not the issue. Some of them are just a hundred feet apart.
The issue is that the closer the runways are, the more separation the planes using those runways must have, and the higher the IFR minima.
For example, below is a picture of Oakland International. The two smaller GA runways (28L and 28R) at the top are close together, but when the field is IFR, only one of them can be used for IFR operations. The runway at the bottom (29) is a commercial runway, but it is far enough from 28L that Oakland can have simultaneous IFR operations on 28L and 29.

This page gives you a pretty good idea of what to expect.

but when you have enough distance, you can land two planes at once. I once was on a flight to Frankfurt, and just seconds before landing, I looked through the right window and saw another plane landing on the parallel runway. After looking at a satellite image, I can assure you that I landed on runway 07L and the other plane landed on 07R.
http://i.imgur.com/mmxS7kr.jpg

yes, that's what I said: "The runway at the bottom (29) is a commercial runway, but it is far enough from 28L that Oakland can have simultaneous IFR operations on 28L and 29"

Heck, you could have simultaneous landings & takeoffs on 28L & 28R at OAK if the airport was under VFR rules (the centerlines are 1000 ft apart.)  I don't know if they ever actually do this, though.

Yes they do simultaneous ops on them, as well as helis to spot C on taxiway Zulu


text:
Is it possible to fly a large aircraft inverted?
I am curious if the inverted plane in the movie Flight 2012 has anything to do with reality / emergency practices. And if it does, can you please explain the concept/ aerodynamics behind of it , or why someone would ever do that even as last resort?

In the movie Flight (2012), Captain Whittaker (Denzel Washington) does not explain why he deliberately inverted his aircraft, but the obvious inference is: since the horizontal stabilizer has slipped off its jackscrew and become jammed into a descent angle beyond the authority of the elevators to correct, in inverted flight this would push the nose far above the horizon and result in either a climb or a stall.
In this attitude, Whittaker would have been pulling back on the yoke to prevent the stall.  In the movie, this worked.  The aircraft was still not under control, but was no longer diving and overspeeding.  Suddenly rolling the aircraft upright in the last seconds before contact with terrain is the unrealistic part.  A hot fighter jet might accomplish this but the roll rate of a passenger jet is too slow.
The movie's flight emergency is based mostly on the crash of Alaska Airlines flight 261 on January 31, 2000.  In that incident, the pilots experienced a jammed horizontal stabilizer with uncontrollable dive, and inverted their MD-80 to arrest their descent.  Unfortunately in real life neither flying officer was stuffed to the gills with booze and coke: the maneouver failed and the aircraft crashed with no survivors.

@Breveleri  a jammed horizontal stabilizer does not leave you with many options? but coke and booze helps?:)

Indeed.  An important plot point of the movie is that a clean and sober pilot would never have attempted such an insane stunt.

It would also be worth mentioning that in real life the fuel feed lines are designed to have gravity in a specific direction. In the Alaska 261 case the engines started spooling down shortly after the inverted position was achieved (see final report)

FedEx 705 underwent extreme maneuvers, even inverted flight. The captain was trying to disorient a hijacker while the first officer fought him hand-to-hand.
Here's a dramatization of it. Not sure if it's the best one but there are others.
Most of your airfoils are designed to produce lift at 0 angle of attack (a perfectly symmetrical airfoil will not), which means that although it's more difficult than with a symmetrical airfoil, you can fly (inverted) at an angle that they will produce lift at negative angles as well. You will be inverted with the nose very high above the horizon, higher than it would be for level flight at a normal attitude.
Beyond that, the aircraft is really not built to do invert maneuvers. Legally they must be able to withstand -1G, but that doesn't leave much room to maneuver while inverted. In reality, most aircraft are designed beyond legal limits just for safety. On top of that, just because an aircraft is rated for a certain load limit does not mean that it will catastrophically fail if that limit is exceeded. It may bend and wrinkle beyond repair, but it should take a lot more than that to fall apart. The problem is that you have no guarantee.

I am aware of the incident and the ability of airliner aircraft to do barrel rolls  , and here you can see some footage http://www.youtube.com/watch?v=AaA7kPfC5Hk my question was a bit different thought.

A barrel roll is a 1G maneuver, it doesn't put any unnatural stress on the airframe if done correctly. FedEx 705 didn't do a barrel roll -at least not a coordinated one- he was inverted.

why would someone do that ? What is the benefit of being inverted. Better gliding ration for the specific airframe ?

There is no benefit. An airliner will not fly well at all at such an attitude. With the huge amount of additional drag, it might not be able to fly level at all. In 705 he was trying to knock the guy with the weapon around long enough for the FO to do something.

Then flight movie i guess is just... movie:)

I would add that Airbus A320 and all later models, Boeing 777 and 787 and some other aircraft won't let you invert them, because their control computer limits pitch and roll.

@JanHudec yeah, talk about taking the fun out of airliner aerobatics ;-)

Boeing 707 barrel roll

Apart from aerodynamics and strength, there are other systems considerations, most notably fuel. The fuel system of most non-aerobatic aircraft is not designed for negative G forces (notwithstanding the fuel pumps etc.) While it will often allow brief excursions to that territory, sustained inverted flight will likely result in fuel starvation or at least unstable fuel supply.


text:
Is it possible to use multiple primary radars to determine an object's location in three-dimensional space?
My question is this: if, for example, I have multiple radars across an area, tracking a particular airborne object, can I use the returns from the radars to determine the geographical location of an object in three-dimensional space (its x-y-z coordinates)?

Of course. That is, after all, the whole purpose of radar: to get the position of an object and plot it on a screen. You don't even need multiple radars, one radar is enough.
If you're just talking about primary radar (i.e. bounce a signal off of the skin of the target), then you're going to have some trouble with measuring the altitude. The distances involved mean that a large change in altitude would create a very small change in the beam angle, so altitude measurements are going to come with huge error bars. In fact, civilian radars almost never bother with vertical scanning at all, because such imprecise measurements aren't considered to be worth the extra complexity. Naturally, secondary radar (i.e. transponder interrogation) doesn't have this issue, since the transponder simply tells the radar what its altitude is.

"If you're just talking about primary radar, then you're going to have some trouble with measuring the altitude." -- Phased array antennas solve this problem.

@TimothyTruckle Not really. The problem is the fact that big changes in altitude create small changes in angle, so no matter what method you use to steer the beam (phased array, moving physical antenna, multiple receiver horns), the error is still going to be there.

"big changes in altitude create small changes in angle" -- Phased array antennas solve this problem too by splitting up the array into 4 sub arrays (2 by 2 matrix) and measuring the phase difference of the signal received between them. This way you can locate an aircraft within the beam horizontal as well as vertical.

@TimothyTruckle Yes, phased array antennas can provide superior resolution. But there is always going to be error, and the practical value of the error is always going to be greater in the vertical. Imagine the angular resolution for some radar is 0.5°. At a range of 50 miles, that's an error of 2300 feet. Horizontally, 2300 feet is nothing. At 200 knots, that's 7 seconds worth of flight time. But, vertically, 2300 feet is a huge value. In congested areas, airplanes are routinely separated by no more than 500 feet. So if you're depending on radar for vertical separation, you're out of luck.


text:
Why is it that only prop planes have mixture?
Prop driven planes have mixture control yet jets don't. Why is this? Even turbine powered props like the SOCATA/Daher TBM have mixture control.

Turboprops don't have a mixture control, they have “condition” control. It shares the function of fuel cutoff, but it is basically a low stop for the power lever. It usually has three positions: high idle, low idle and off. High idle sets the idle, i.e. low stop of power to setting that can maintain full prop RPM (because you don't want to go lower on approach), low idle sets it to minimum that will keep the engine running (for idling on ground) and cut off will turn the engine off. But it's not an independent control.

RE -- TBM -- https://www.nasflmuseum.com/the-avenger.html -- not a turboprop.  Please clarify in a way that is not dependent on an internet link.

Seems like OP is referring to the Daher TBM series, not the WWII bomber.

I submitted an edit to clarify the aircraft.

..... Because it's hard to get any substantial amount of thrust out of a piston engine exhaust ?

Spark-ignition (gasoline) piston engines have separate throttle and mixture controls. This is because power of spark-ignition piston engines is regulated by throttling the air intake, into which the fuel is mixed. The correct fuel/air ratio is ensured by metering the air using the carburettor or with a similar sensor for fuel injection, but for various reasons adjustments of the ratio need to be done, so there is a mixture control (that may be automated with electronic control unit).
Other kinds of engines – diesel and turbine – don't meter air. They always induce all the air they can and burn the fuel lean, in excess air. Therefore they only have one thing to control, the fuel valve.

The difference is because spark-ignited engines (Otto cycle) mix the fuel with air in the intake manifold and then ignite it with spark-plugs. And such mixture will only ignite if it is sufficiently close to the stoichiometric ratio, the ratio where all the fuel just exactly reacts with all the available oxygen. If there is too much air or too much fuel, the excess will absorb the heat, preventing the mixture from heating up enough and the flame won't propagate. So the engine must induce the right mixture, and its pressure is restricted to reduce power.
In contrast Diesel (Diesel cycle) and turbine (Brayton cycle) engines induce air at whatever pressure is available and inject the fuel into the compressed air at a point where it will auto-ignite at the point the spray from the nozzle sufficiently mixes with air – because the air is hot enough from the compression alone in Diesels and because the flame is continuous in turbines. So there is no problem with having excess air and therefore the air does not have to be metered. Only the fuel is metered, so there is only one lever.
Older spark-ignition engines use carburettor. This induces the fuel simply due to drop in pressure created by a venturi in the intake manifold. But they have to be adjusted with altitude. Best explanation I found is that density of air decreases with pressure, but density of the fuel does not, so while the fuel bowl feels the correct pressure, it still injects relatively more fuel as the air density decreases. This requires adjusting the mixture as the aircraft climbs, otherwise it would become too rich and the engine would stall.
Injected engines, including those with “pressure carburettors” that are simple kind of indirect injection, handle this correctly, but you still want to use richer mixture for high power as the engine runs cooler with a bit of excess fuel, or a leaner mixture for efficiency, as the engine has less power, but higher efficiency with a little bit of excess air. So injected engines still have mixture control.
Only engines with electronic control (ECU; also called FADEC, full authority digital engine computer) automate this completely (using richer mixture at full power), but that's rare in piston airplane engines with exception of some experimental (home-built) aircraft with adapted automobile engines.
Newer turbines and diesels also usually have FADEC, but that is mainly providing protection from stalling or overheating the engine with careless power manipulation. And older turbines already had FCU, a simpler and not as good electro-mechanical system with the same purpose. But there is still only one variable it controls, not two like in spark-ignition engines.

Pre-mixing air and fuel in a way that requires manual regulation is only a thing in older, mostly carbureted gasoline piston engines. Most uses have replaced this method by fuel injection in late 20th century.
Automated mixture control for carbureted engines — feedback — was attempted, but wasn't very reliable. With fuel injection, mixture control could be easily automated, and most injection engines have an automated system to manage the fuel:air ratio.
Manually controlled engines have persisted in general aviation due to regulation driving up the cost of new designs. New aircraft designs, such as most LSA (Light Sports Aircraft), also tend to drop manual mixture control.
Diesels, turboprops, and jets supply clean air to the engine, and inject the fuel separately. In a jet, your throttle is your mixture control.

I see the point here, but there is little difference between curburetor and port injection in terms of fuel delivery in the sense that both deliver a pre-mixed charge to a cylinder. Direct injection engines are another thing. The way the mixture is regulated is the real difference, archaic aero engines lacking any smarts in that area, regardless of being cabureted or injected.

LSA don't have manual mixture control because they predominantly use Rotax 912 (series) engines, see this question

Gasoline piston engines with fuel injection (such as the popular IO-360) still have a mixture control. It’s not just about carburetors.

@StephenS Right, corrected.

I'm just a layman when it comes to fuel mixing, but from everything I have seen, the fuel injection people talk about precision measurements more.  The carburetor people seem to have a lot more art to what they do.  There's more "feel" to it.  I can definitely see why automation of fuel injection mixtures is much more of a thing than automated carburetor mixtures are.

In a piston engine, each cycle of the piston draws in a certain volume of air in which a certain volume of fuel is to be burned, adding heat to the gases in the cylinder. In the ideal case, that quantity of air is only sufficient to burn to completion the volume of fuel mixed in with it on each cycle; that is, the fuel-air mixture is ideally chemically stoichiometric (no excess air, no excess fuel).
In practice, deviations from perfect stoichiometry are dialed in to keep the engine happy (excess fuel in a rich mixture to carry off heat and prevent overheating the heads) or to maximize efficiency ("xxx degrees F lean of peak" operation). The pilot is responsible for managing that mixture with a mixture control.
But in a turbojet (for example) running not on the Otto cycle but on the Brayton cycle, there is always excess air flowing through the engine, to which a variable flow rate of fuel is injected and burned in the combustor cans. More fuel flow means more oxygen gets consumed as needed out of the excess air flowing through the engine, adding more heat to the gas and hence more thrust and a higher rotation speed of the engine, but at the end of the combustion process the exhaust always contains unconsumed oxygen because the engine's combustion process is carried out in an excess-oxygen environment. This is why you can get a sudden increase in thrust by dumping more fuel straight into the hot gas downstream of the power turbine disc before ejecting it out the exhaust nozzle i.e., afterburning.
Now if you tried doing that fuel dump into the combustor cans instead to consume all the oxygen available, then the turbine temperature would promptly go through the roof and the blades would melt!
So, there is no mixture control as such operable by the pilot of a turbojet.
Note that if a turbojet ran like a gasoline Otto-cycle engine, there would be almost no unconsumed oxygen contained in the exhaust gas stream and afterburning would be impossible.
See Jan Hudec's answer below for more detail about how all this works.

Diesel is also a piston engine.

@JanHudec, thanks, will edit. -NN

You've explained why a gas turbine doesn't have to operate at stoichiometric. The only thing I'd add is to explain that it can't operate at stoichiometric, because the combustion temperature would be so high that the turbine blades would melt. This isn't a problem in piston engines because the cylinders only see the average cycle temperature, whereas in a gas turbine, the turbine blades see the combustion temperature continuously.

@LevelRiverSt, will edit. -NN

There is no need for mixture control in a jet.  In a FADEC engine, (Full Authority Digital Engine Control) the computer meters the fuel.  In a non-FADEC turbine engine the FCU, (Fuel Control Unit) is an electro-mechanical device that meters fuel according to inputs such as throttle position, inlet temperature, burner pressure, etc.
I think you are mistaken about turboprops having mixture control.  I haven't flow a turbine TBM, but neither the C-208 or T-34 both with PT6 turbines had a mixture control.  The lever you are thinking about is probably the fuel condition lever.  It looks similar, but performs a different function.

It has nothing to do with automation. Turbine only has one input to control, the fuel, so there is simply no need for another lever.

@JanHudec, I didn’t say anything about automation other than casually mentioning FADEC.  Old FCU engines also have just one lever, as I stated.

My point is that the presence of FCU or FADEC is completely besides the point here – the main reason is that the fuel flow is the only controlled input, whether via FADEC, FCU or directly (at least Kodiak has option for direct control for case the FADEC fails, the pilot just has to closely watch the temperature), so there is simply no use for a second lever.

@JanHudec but it is not beside the point.  It is just as relevant as mentioning carburetor, throttle, or fuel injection in this context.  Because the pilot does NOT manually meter the fuel with the power lever.  Unless, as you point out, the FCU fails and you have to use the EPL.

Thank you everyone! All of these answers and comments have fully answered my questions.

The question is not that prop planes have mixture controls, but that piston engines usually come with them.
The reason is exceedingly simple. Aircraft engines manage fuel/air mixture using a system that is very unlikely to fail catastrophically: the pilot. There is no reason why a computer couldn't take over the function, except the excessively convoluted certification process (for many good reasons) that makes it cost prohibitive.
There have been single-lever aircraft piston engines out there (the WW2 era Fw190 comes to mind as an early example), but far too often they have not withstood the test of market conditions and time. Today a few production piston aircraft come with the necessary electronics to forgo the mixture control.
The reason for turbine engines automating those controls is that the tolerances for acceptable fuel/air mixtures are much tighter. In a piston engine key components are operating hundreds of degrees below their melting point. By comparison, a turbine engine might only be 50 degrees short of where things start to dramatically change their ductility. While theoretically a human might be able to manage the fuel/air mixture of a turbine engine as a full time job, the probability of human error causing expensive engine damage (or worse) is just too high. Hence the automation.

The Diamond planes have only one engine control lever.

... and the human factor is on its way out of piston engine mixture control, modern ECU's outperform humans in both accuracy and reliability.

@Jpe61 Doesn't even need to be modern; car ECUs achieved this perfectly well in the early/mid-90s when fuel injection became mainstream.  It depends whether you mean "modern" to be within 10 years or 100 years. :)

@Koyovis, most Diamond aircraft use Diesel engines. They do have FADEC too, but that isn't the reason – the reason is that Diesels only have one input to control.

It also isn't about automation. Turbines always have some kind of electro-mechanical fuel control, otherwise they'd be too hard to manage, but the main reason is that they simply only have one input, the amount of fuel, so there is no need for a second lever.

"Aircraft engines manage fuel/air mixture using a system that is very unlikely to fail catastrophically: the pilot." I'm not sure that modern aviation statistics would agree that a pilot is significantly less likely to fail catastrophically than automated systems. :P

This answer claims the tolerances for acceptable fuel/air mixtures are much tighter (in turbines than in gasoline piston engines).  But that's basically the opposite of what @JanHudec's answer explains.  It's not that the ratio of fuel to air is must be within a narrow range at all.  There's a wide range between melting the engine and letting the flame go out, and any fuel flow setting in that range is fine, isn't it?  Excess air isn't a problem. FADEC or FCU set an upper limit on fuel flow for the operating conditions (right?), but the tight tolerance is operating at max power w/o damage?

@PeterCordes, indeed, the last paragraph is utter nonsense. Turbines and diesels burn lean, so they don't control mixture at all. Turbines have fuel control units to reduce the risk of exceeding the temperature limit, but even without it there is still just one control, not separate throttle and mixture.

Tough crowd here. However I do suggest that you are too close to the trees to see the forest. Here's a thought experiment about acceptable fuel/air mixtures: let's take an aviation gasoline piston engine with a throttle stuck full open. Except for some damage from detonation, almost any mixture setting will be acceptable between full power and poor running due to fuel starvation. Now imagine a turbine engine with a manual fuel valve, and consider the probability that a human can set a desired power level and maintain it without some sort of automation. (To be continued)

And how likely would it be that such manually controlled turbine engine tears itself apart or flames out? Now as to throttles in diesel piston engines, it's obvious y'all haven't actually worked with them. Plenty of diesels have throttles, or use some other mechanism to control air flow. I just recently tore one down that has a throttle body. Yet other diesel pistons use a variable turbo and turbo bypass to achieve the same effect as a throttle.


text:
Do any flight simulators go completely inverted?
From the pictures I have seen of airline simulators, it looks like the range of motion is limited to somewhat level flight. Are there any flight simulators, possibly military trainers, that can do a complete barrel roll?

Everything I can find says no. The ones that make you feel upside down work on your inner ear and often don't rotate more than 90 either way, if that. (I'm in military flight training, and the ones we have for the trainers can't go inverted, even though the planes themselves can)

Do note also that the simulator motion does not reflect the aircraft motion. What it does instead is simulate acceleration. Imagine that you are accelerating in a car, you feel pressure in your back. In a simulator, when you increase speed, the simulator tilts nose up so that you have more pressure on your back.

I've never really understood how simulators simulate G-force anyway-- if they do. G-force at the top of a barrel roll might be just 1-G in the normal upright direction in the pilot's reference frame.  Well that's easy.  Still it seems if you are trying to simulate G-load AND sensations related to rotational accelerations, soon or later you are going to run out of options for freedom of motion unless you are replicating the aircraft's ACTUAL motion through space.

Re second comment above-- interesting-- yet it seems the rotation associated with tilting the nose up might be detectable and confusing--

Pulseworks makes some full motion sims that are capable of inverted flight and infinite barrel rolls.  These sims fall into the entertainment category more than the training category, but they are full motion, in cockpit sims.  I have also been in a similar type of sim at a Harris/Boeing sponsored party in Seattle made by a company whose name I cannot remember that did the same thing.  That one in fact was capable not only of rolls but also loops and could be pretty violent (I wanted to see if it would spin... it did).
Here's a video of one of the Pulseworks models.  They have some at the Udvar-Hazy center at IAD (free admission, and a shuttle from the terminal).  They cost a bit of money to get into.  They are also often setup in pairs so you can go into a dogfighting scenario with the box next to you.  Bring a friend.

Wow, duh! I've seen these. Totally forgot about them

They also have them at the main building of the Air and Space Museum (downtown DC, also free admission, also costs $8 for the sim). Well worth it.

It is possible to design one which does but impractical.
The PulseWorks simulator, while entertaining, is not realistic and cannot accurately simulate inverted flight.  The simulators motion is not made so the sim cabin matches the actual attitude of the real aircraft during maneuvers; rather to induce sensory illusions in the trainee crew which match what one would expect in flight.
Full Motion Simulators (FMS) are used, primarily for transition and currency training on large aircraft which operate in the air transport category and where the training is simply impractical for both cost and safety reasons to do it in the real thing.  They are designed to create motion which can mimic sensory illusions during flight in the +1-1.3 G range, typical for all commercial air carrier or cargo operations.  As such building a simulator with full inverted capability would simply be unwarranted for the requirements of a typical FMS.  It also provides a pretty inaccurate simulation of the kinesthetics of inverted flight, as most inverted condition end up being in the positive G range.  It's also not practical for aerobatic or military flight training as at most it could only simulate -1G and -1-2G for a brief moment, and the whole simulator module would have to be rolled inverted for the effect even when simulating negative G maneuvers while the aircraft is erect.

Shouldn't it be pretty easy to build a simulator that accurately simulates large positive and negative Gs by centrifuging the simulator pod about a horizontal axis?

@Sean: Possible, yes. Easy, no.

Like it has been said here it is very difficult to simulate g-forces outside of the common civil flight envelope.
But it is possible! I have built one, you can watch videos of it on YouTube here, and here.
Like Zeus said, even with this 2DOF 360° you will feel "wrong" g's very often, e.g., banking with 90° feels like a 1 g rudder slide. And a loop feels more like flying inverted (-1 g). But in real life you have +3 g's.
I think this simulator is very useful to practice aerobatics!
You feel the same emotions, fear, and excitement, like in real life, e.g., my first real life loop scared me in the same way the simulator did. Same thing with barrel rolls, lazy eights, cubans, hammer heads, inverted, etc.
Also it is very exhausting and your stomach will have some fun for sure :-D Flying with a VR headset teaches the right reactions very, very well. Actually I make, or do not make, the same mistakes both in the simulator and in real life.
A friend (a Lufthansa A320 pilot) was also surprised how good this thing is in teaching, but to reach this level the simulator requires many little (invisible) additions: sound shakers, g-effects (on set cueing), VR headset, g-belt tensioner, force feedback, and many things more.
In comparison to standard simulators, this concept is far advanced for aerobatics or military training, IMHO.

I believe there was one at RAFC Cranwell that could go inverted. Source: my sister's, at the time, fiancée who was training there. I may be remembering wrong - it was a while ago.
Also, while not a flight sim in itself, many years ago there was an arcade cabinet for Sega's G-LOC called the R360 that had full 360 degree motion about all axis. I would be gob-smacked if that same tech had not been used in more professional environments.

For professional simulation, fidelity is paramount. It's better to have no motion than to have wrong motion that produces negative learning. Motion is not for fun; it's an actual feedback channel that pilots use. And as it happens, it's nearly impossible to provide reasonable fidelity with full roll/pitch rotation (for an airplane), except for, perhaps, very special isolated cases. (By the way, full yaw rotation can actually be beneficial - and is easier to implement. Yet it doesn't look like Sega R360 can do yaw.)